[clang] [llvm] [cmake] Add config for riscv(32|64)-unknown-elf (PR #176353)

[Ramkumar Ramachandra via cfe-commits]

artagnon wrote:

> > the inclusion of LLVMExports relies on the target system supporting shared libraries
> 
> But you're building the compiler for the host, not the target, so why does that matter?

Yes, the issue is that I get CMake errors when building compiler-rt (in its cmake:112, we call load_llvm_config()), because I set the SYSTEM_NAME while building it to Generic?

https://github.com/llvm/llvm-project/pull/176353